Recently, the housings of portable terminal devices typified by cellular phones, smartphones, and personal digital assistants (PDAs) have been required to have reduced dimensions and weight from the viewpoint of compactness and lightness. Accordingly, demands have arisen for more compact antenna devices. It has also been required to allow a single portable terminal device to communicate with a plurality of radio systems using different frequency bands.
Under these circumstances, conventionally, for example, a multiply resonant antenna device has been proposed, which is designed to implement multiple resonance by combining a monopole element having a plate-like design formed on the feed point side with a parasitic element. In addition, there has been proposed a multiply resonant antenna device which is obtained by combining a monopole element with a folded monopole element and providing a parasitic element in a direction opposite to the monopole elements. There has also been proposed an antenna device which includes an antenna plate having an inverted trapezoidal design provided for a feed point, and generates a plurality of resonant frequencies by using a short side and lateral side of the antenna plate as radiative elements.
The conventionally proposed antenna devices have the following problems. That is, as the antenna device obtained by combining the monopole element and the parasitic element is reduced in profile, the distance between the monopole element and the ground pattern decreases, resulting in a decrease in antenna impedance at the feed point. In addition, depending on the resonant frequency of the parasitic element, parallel resonance occurs between two resonant frequencies generated by the monopole element. This leads to deterioration in radiative efficiency.
The antenna device including the monopole element, folded monopole element, and parasitic element is configured to make the folded monopole element generate the lowest resonant frequency, and hence the folded monopole element has a long element length, resulting in an increase in the size of the antenna device. In addition, since the parasitic element has a small influence on the monopole element and the folded monopole element, it is difficult to implement a continuous wide band.
An antenna plate having an inverted trapezoidal design provided for the feed point requires a large area for installation. It is therefore difficult to reduce the size of the corresponding antenna device. In addition, this device exhibits low radiative efficiency at a resonant frequency as compared with a general multiply resonant antenna independently provided with an antenna element.